What wire size for 40 A continuous, 180 ft, 240 V single-phase?
Ampacity says #8 Cu, voltage drop forces #6 Cu. Aluminum equivalent: #2. Full NEC 310.16 + 210.19 calculation, the EV-charger case (where this comes up most), and the conduit-fill check.
Short answer: for a 40 A continuous load at 180 ft on 240 V single-phase, ampacity alone allows #8 AWG copper at 75°C (per NEC 310.16) once you derate for continuous load — but voltage drop forces you bigger. Use #6 AWG copper THHN/THWN minimum, or upsize to #4 copper for safety margin. Aluminum equivalent: #2 AL (USE-2/SE/SER). Below is the NEC 210.19 reasoning, both calculations (ampacity AND voltage drop), and the two ways most people get this wrong.
Step 1 — Ampacity (NEC 310.16)
Continuous load = 40 A NEC 210.19(A): branch-circuit conductors must carry 125% of continuous load. Required ampacity = 40 × 1.25 = 50 A NEC 310.16, 75°C copper column: #10 AWG → 35 A ← FAIL (below 50 A) #8 AWG → 50 A ← MEETS exactly #6 AWG → 65 A ← passes with margin For aluminum (75°C): #8 AL → 40 A ← FAIL #6 AL → 50 A ← MEETS exactly #4 AL → 65 A ← passes with margin Take the smaller, with margin: #8 Cu or #6 AL.
Step 2 — Voltage drop check
Single-phase voltage drop:
VD = 2 × K × I × L / cmil
K = conductor resistivity (Ω-cmil/ft):
Copper: 12.9 (75°C)
Aluminum: 21.2 (75°C)
I = current in amps
L = one-way length in feet
cmil = circular mils of the conductor (NEC Ch 9 Table 8)
cmil reference:
#10 AWG = 10,380
#8 AWG = 16,510
#6 AWG = 26,240
#4 AWG = 41,740
#3 AWG = 52,620
#2 AWG = 66,360
1/0 = 105,600
2/0 = 133,100
3/0 = 167,800
4/0 = 211,600Worked example: 40 A, 180 ft, 240 V, copper
Voltage drop budget (3% of 240 V): 7.2 V
#8 Cu:
VD = 2 × 12.9 × 40 × 180 / 16,510
= 185,760 / 16,510
= 11.25 V → 4.69% on 240 V FAIL
#6 Cu:
VD = 185,760 / 26,240
= 7.08 V → 2.95% ✓
#4 Cu:
VD = 185,760 / 41,740
= 4.45 V → 1.85% ✓ (more headroom)
Verdict (copper):
#8 Cu meets ampacity but FAILS the 3% VD check at 180 ft.
#6 Cu THHN/THWN is the minimum to satisfy both.
#4 Cu gives ~40% margin if the load grows or a future
inspector enforces tighter VD.Worked example: 40 A, 180 ft, 240 V, aluminum
Voltage drop budget (3% of 240 V): 7.2 V
#6 AL:
VD = 2 × 21.2 × 40 × 180 / 26,240
= 305,280 / 26,240
= 11.63 V → 4.85% FAIL
#4 AL:
VD = 305,280 / 41,740
= 7.31 V → 3.05% borderline (just over 3%)
#2 AL:
VD = 305,280 / 66,360
= 4.60 V → 1.92% ✓
Verdict (aluminum):
#6 AL meets ampacity but FAILS VD.
#4 AL is borderline — passes ampacity, just over 3% VD.
#2 AL is the safe pick. Many residential 50–60 A subpanels at
this distance run #2 AL SER for this exact reason.The two checks side by side
Wire Ampacity (75°C) VD at 40A 180ft Verdict ───────── ─────────────── ────────────── ─────────── #10 Cu 35 A 18 V (7.5%) FAIL ampacity AND VD #8 Cu 50 A 11.25 V (4.7%) Pass ampacity, FAIL VD #6 Cu 65 A 7.08 V (2.95%) PASS both ← min #4 Cu 85 A 4.45 V (1.85%) PASS both, margin #3 Cu 100 A 3.53 V (1.47%) Overkill #8 AL 40 A 18.4 V (7.7%) FAIL both #6 AL 50 A 11.63 V (4.85%) Pass amp., FAIL VD #4 AL 65 A 7.31 V (3.05%) PASS amp., borderline VD #2 AL 90 A 4.60 V (1.92%) PASS both ← min AL #1/0 AL 120 A 2.89 V (1.20%) Overkill
Why 3% (and where the rule comes from)
NEC 210.19(A)(1) Informational Note 4 recommends branch-circuit conductors be sized so voltage drop doesn't exceed 3% at the outlet farthest from the source, with combined feeder + branch ≤5%. Informational Notes are NOT enforceable code — they're recommendations. However:
- Many states (e.g., California Title 24) enforce 3% / 5% by adoption.
- Most commercial spec writers and consulting engineers specify 3% / 5% on drawings.
- If the load is a motor, EV charger, or sensitive electronics, 3% is the ceiling, not the target — voltage drop affects torque, charge time, and equipment life.
- If the inspector flags it, an "Informational Note" argument doesn't hold — the engineer of record's spec or the AHJ's adopted code rules.
40 A continuous: where this load comes up
- Level 2 EV charger, 50 A circuit: continuous draw 40 A. Most common case.
- Tankless water heater (60 A model): actually pulls ~48 A continuous — need bigger wire than 40 A example.
- Subpanel feeder: 50 A feeder with continuous loads on the load side.
- Detached garage 240 V circuit: 50 A breaker, ~40 A continuous if it has a welder + lights running.
- Heat pump compressor (small unit): 40 A MOCP (Maximum Overcurrent Protection).
Conduit fill check (relevant if running in conduit)
Run: 2 hot + 1 neutral + 1 ground in EMT for 240 V single-phase
residential subpanel feeder.
Wires (THHN):
2 × #6 Cu hot: 2 × 0.0507 sq in = 0.1014
1 × #6 Cu neutral: 1 × 0.0507 = 0.0507
1 × #10 Cu ground: 1 × 0.0211 = 0.0211
Total: 0.1732 sq in
EMT internal area & 40% fill (NEC Ch 9 Table 4):
3/4" EMT: 0.533 sq in × 0.40 = 0.213 sq in ✓ (#6 fits, 32% fill)
1" EMT: 0.864 sq in × 0.40 = 0.346 sq in comfortable (20% fill)
For #4 Cu (heavier wires):
2 × 0.0824 = 0.1648
1 × 0.0824 = 0.0824
1 × 0.0211 = 0.0211
Total: 0.2683 sq in
3/4" EMT: NO — exceeds 40% (0.213 sq in limit)
1" EMT: YES — 31% fillDon't forget the OCPD
Even with #6 Cu wire that's rated for 65 A, the breaker is sized to the load — not the conductor. For a 40 A continuous load, per NEC 210.20(A): the OCPD must be ≥125% of continuous load = 50 A. So:
- Conductor: #6 Cu (rated 65 A, derated 65/125% = 52 A continuous, or 65 A non-continuous).
- Breaker: 50 A (sized to 125% of 40 A continuous load).
- Standard breaker sizes (NEC 240.6): 50 A is standard. Don't round up to 60 A unless the load actually justifies it — oversized OCPD can be a code violation if the conductor doesn't carry it.
Common mistakes
- Sizing on ampacity only, ignoring VD. The wire passes 310.16 but the EV charger trickles overnight because of voltage drop on a 200 ft run.
- Using K = 11 for copper at 90°C. The 75°C value (K = 12.9) is what most installations actually run at, and what 310.16 references for THWN-2/THHN.
- Mixing aluminum and copper at terminations. Most modern panels accept aluminum on the dual-rated lug; old single-rated copper-only lugs are not OK with aluminum.
- Forgetting the 125% continuous-load multiplier. Computer load = 40 A → conductor & OCPD sized for 50 A, period.
- Pulling #4 Cu in 3/4" EMT. Math says no (over 40% fill). Always check Chapter 9 Table 4.
The recipe in plain English
40 A continuous, 240 V single-phase, 180 ft one-way: Copper THHN/THWN-2: #6 minimum, #4 preferred Aluminum SE / SER: #2 (best fit for VD + ampacity) Breaker / OCPD: 50 A (125% of 40 A continuous) Conduit: 1" EMT for #6 + ground (or #4 with margin) Equipment ground: #10 Cu (NEC 250.122)
Run it on your phone
The ElectricianCalc app sizes conductors for ampacity (310.16 + derating) AND voltage drop simultaneously, so you don't have to run two separate calculations. Type the load, distance, voltage, and wire material; it returns the minimum size that passes both checks. 100% offline. Free on the App Store and Google Play.
Related
- Conduit fill: 4× #10 + 1× #6 THHN in 3/4" EMT
- Voltage drop and NEC 210.19(A) explained
- NEC wire sizing for branch circuits (full reference)
- ElectricianCalc — NEC calculator on iOS + Android
Note: NEC interpretations vary by AHJ and code year adopted. Always verify against the code edition your jurisdiction has adopted (some states are still on NEC 2017 or 2020). Defer to the engineer of record on commercial work.